1. Field of the Invention
This invention relates to a Group III-V compound semiconductor, and in particular to a semiconductor light-emitting element and also to a method of manufacturing the semiconductor light-emitting element.
2. Description of the Related Art
The GaN and GaN-based mixed crystals, each representing one of the Group III-V compound semiconductor containing nitrogen, are large in band gap, i.e. 3.4 eV or more and hence are of direct transition type, so that these mixed crystals are expected to be useful as a material for a short wavelength semiconductor laser element. For example, a bright blue LED exhibiting a luminance of the order of 1 candela has been realized by making use of these mixed crystals. However, as far as a semiconductor laser is concerned, the use of these mixed crystals so far reported is limited to those which is capable of performing a pulse operation based on an optical excitation. Namely, up to date, it has been failed to realize a semiconductor laser which is capable of performing a laser operation through the injection of current.
As for the structure of light-emitting element now studied, a hetrojunction where GaN or GaInN containing a small content of In is employed as an active layer for generating light and AlGaN is employed as a confinement layer for confining light and electrons is considered to be promising. In order to obtain a semiconductor laser which is capable of effecting a continuous operation at room temperature, the film thickness of the confinement layer is required to be sufficiently thick so as to achieve sufficiently high confinement effects. Furthermore, a difference in forbidden band width between a light-emitting layer and a confinement layer is required to be sufficiently large and at the same time a heterojunction having a flat interface is required to be formed.
According to the studies made by the present inventors, the mole fraction (x) of Al in a composition of Al.sub.x Ga.sub.1-x N is required to be at least 0.15, preferably 0.25 to 1.0. In view of mismatching in lattice between the confinement layer and the active layer, the upper limit of the mole fraction (x) should desirably be 0.5 or less. Moreover, as far as the range of wavelength from blue to near ultraviolet ray is concerned, the film thickness of the confinement layer should be at least 0.3 .mu.m, preferably in the range of from 0.5 .mu.m to 1 .mu.m.
However, it is fundamentally very difficult according to the conventional technique to form a flat and thick AlGaN layer having a high Al content. Since a substrate of high quality which is capable of matching in lattice with a GaN-based material has been failed to be found so far, a sapphire has been extensively employed as a substrate for forming GaN thereon. However, since the mismatching in lattice between sapphire and the GaN is as large as 15% or so, the GaN is more likely to be grown in the shape of an island.
In an attempt to alleviate any influence from the mismatching in lattice between sapphire and the GaN, a method of forming an AlGaN layer on the surface of a buffer layer formed in advance on a substrate has been employed. According to this method, a very thin amorphous or polycrystalline AlN or GaN film is formed as a buffer layer on a sapphire substrate by way of a low temperature growth. In this case, since the amorphous or polycrystalline film functions to alleviate any thermal strain, the fine crystals included in the interior of the buffer layer would become seed crystals which are uniform in crystal orientation at the occasion of heating up to 1,000.degree. C. As a result, the quality of crystal of the AlGaN layer formed on the buffer layer is considered to be improved.
When the aforementioned method is employed, the quality of crystal represented for example by a half-value width of X-ray diffraction depends greatly on the conditions of growing the buffer layer constituting an underlying layer. For example, if the buffer layer is relatively thick, the orientation of the seed crystals constituting nuclei for crystal growth may be disordered, so that the quality of crystal of the AlGaN layer formed on the buffer layer would be deteriorated. The aforementioned half-value width may be decreased as the thickness of the buffer layer becomes thinner. However, when the film thickness of the buffer layer is 10 .mu.m or less, the surface conditions of the crystal may become abruptly deteriorated.
As explained above, the conventional method is defective in that the crystal of a compound semiconductor layer that has been grown directly on the buffer layer is more likely to be poor in quality, and it is difficult to obtain a compound semiconductor layer, particularly in the case of AlGaN, having a half-width value of not more than one minute in the X-ray diffraction. If the film thickness of the AlGaN layer is increased in view of improving the quality thereof, cracks may be generated in the AlGaN layer. These phenomena are more conspicuous in the case of the AlGaN to be used as a confinement layer.
FIG. 1 represents the relationship between the molar fraction of Al in Al.sub.x Ga.sub.1-x N and the maximum film thickness which enables the growth of crystal layer to be effected without giving rise to the generation of cracks. According to the aforementioned conventional method, if an Al.sub.x Ga.sub.1-x N layer having a film thickness of not less than 0.3 .mu.m is to be formed without inviting cracks, the molar fraction of Al (x) is required to be restricted to less than 0.15. As a result, the growth conditions of the buffer layer will be extremely restricted. Furthermore, the quality of the crystal of the buffer layer obtained in this manner is also not excellent. These problems have been main obstacles to the manufacture of a bright light-emitting diode or a semiconductor laser requiring as a confinement layer the employment of an AlGaN of high quality and large film thickness.
The reason for employing an amorphous or polycrystalline material for a buffer layer consisting of AlN or GaN in the conventional method is due to the fact that NH.sub.3 gas is employed as a nitrogen source. Namely, in the growth of the buffer layer using NH.sub.3 gas, the growth is required to be carried out at a low temperature of less than 600.degree. C. in order to inhibit the reaction between the NH.sub.3 gas and the substrate. Therefore, the resultant layer formed under such a low temperature condition inevitably becomes amorphous or polycrystalline. It has been considered that even if the buffer layer is to be formed in the form of monocrystal, the quality of the resultant monocrystal would be badly deteriorated so that a layer to be grown on such a buffer layer would become rather poor in quality of crystal.
As explained above, it has been very difficult up to date to grow in good reproducibility an AlGaIn-based semiconductor layer of excellent quality and sufficient thickness on a substrate which mismatches in lattice with the AlGaIn-based semiconductor layer. This has been a cause of deteriorating the yield in the manufacture of a bright short wavelength light-emitting element or a short wavelength semiconductor laser.